Analysis of Strain Accumulation of the Faulting Zones by the Help of Continuous GPS Stations

Analysis of Strain Accumulation of the Faulting Zones by the Help of Continuous GPS Stations Asude A. SØRENSEN, Norway and Onur YILMAZ, Turkey Key wor...
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Analysis of Strain Accumulation of the Faulting Zones by the Help of Continuous GPS Stations Asude A. SØRENSEN, Norway and Onur YILMAZ, Turkey Key words: Strain measurements, GPS, Continuous GPS Network. SUMMARY The North Anatolian Fault Zone (NAFZ) belongs to one of the largest recent active fault systems around the Earth. During the last decades a westward expansion of large earthquakes along the North Anatolian Fault has shown an extension around Marmara Sea. The researchers give attention to the seismic gap in the Marmara Sea. It is expected that northern and middle strand of NAFZ have a strong risk for future actions. To be able to measure displacements and crustal deformations, a series of continuously operating GPS stations were installed around the Marmara Sea, Turkey, by TUBITAK. This network is named as Marmara Continuous GPS Network (MAGNET). The purpose of this study is to estimate velocity field of the region, and to investigate changes of the strain accumulation in the region by using seven MAGNET stations around the Marmara Sea. Velocity field estimation is done by using GIPSY-OASIS Software and strain accumulation investigation is done by triangulation method and coordinate differences method. Since there are two main goals of this study, then there are two main results. From the seven MAGNET stations, with GIPSY-OASIS Software, the displacement of the stations between the years of 2002 and 2004 is found to be 24±1 mm/year. Also the strain accumulation of the sub-regions around the Marmara Sea is determined as either extension or compression. OZET Kuzey Anadolu Fay Kusagi Dunyanin en genis aktif fay sistemleri arasinda bulunmaktadir ve Anadolu Plakasinin en etkili kusagidir. Son yillarda Kuzey Anadolu Fay Kusagi boyunca bati yonundeki genislemeler Marmara Denizine kadar ulasmaktadir. Arastirmacilar Marmara Denizi icerisindeki sismik bosluga dikkat cekmektedirler. Cunku bu bosluk KAFZ’un kuzey ve orta bolgelerinde gelecek icin buyuk bir risk tasimaktadir. Bu calismanin iki amaci vardir. Birincisi bolgedeki hiz alaninin belirlenmesi ve ikincisi ise yedi sabit GPS istasyonundan alinan verilerle gerinim analizinin hesaplanmasidir. Bu arastirma ucgenleme ve koordinat farklari alinarak yapilmistir. Gerinim parametreleri; maksimm.minimum asal gerilme, maksimum-minimum gerinim parametreli donuklugu, maksimum kayme gerinimi olarak tanimlanmistir.

TS 3I - Deformation Monitoring – Techniques and Case Studies Asude A. Sørensen and Onur Yilmaz Analysis of Strain Accumulation of the Faulting Zones by the Help of Continuous GPS Stations Integrating Generations FIG Working Week 2008 Stockholm, Sweden 14-19 June 2008

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Analysis of Strain Accumulation of the Faulting Zones by the Help of Continuous GPS Stations Asude A. SØRENSEN, Norway and Onur YILMAZ, Turkey 1. INTRODUCTION Global Positioning System (GPS) has been a very useful tool for the last two decades in the area of geodynamics. Its high accuracy in determination of relative positioning availability together with its modest budget, increases the use of GPS all around the world. GPS measurements are used to determine the strain accumulation along fault lines. Moreover, GPS measurements provide a significant tool for determining tectonic strain rates, which are assumed to be indicative of earthquake potential (Jackson, D.D et al., 1999). GPS measurements can be analyzed in two main methods as continuous and campaign measurements. The Marmara Continuous GPS network (MAGNET) was established to measure the deformations associated with strain accumulation along the western NAF system. In this study, firstly the velocity vectors of seven MAGNET stations which are located in western Marmara were determined. After that, the strain accumulation around the fault zone at western Marmara was calculated. 2. MARMARA REGION NAF is a relatively simple, narrow, right-lateral strike-slip fault zone. NAF splits into several fault strands in the vicinity of the Sea of Marmara so that the deformation (surface faulting of the NAF) becomes distributed over a 120 km broad zone. The region of the Marmara Sea is a transition zone between the strike slip regime of the NAF and the extension regime of the Aegean Sea. The main branch of the dextral strike-slip North Anatolian fault enters to Marmara Sea through the Gulf of Izmit to the east and comes out through the Ganos fault, north of the Gallipoli peninsula, to the west (Fig.1).

TS 3I - Deformation Monitoring – Techniques and Case Studies Asude A. Sørensen and Onur Yilmaz Analysis of Strain Accumulation of the Faulting Zones by the Help of Continuous GPS Stations Integrating Generations FIG Working Week 2008 Stockholm, Sweden 14-19 June 2008

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Figure 1: The Marmara region showing the mapped faults (Barka et al., 1988)

3. DATA ANALYSIS with GIPSY The software used for the processing of the GPS data is GIPSY OASIS II (GPS Inferred Positioning SYstem Orbit Analysis and Simulation Software), (Gregorious, 1996) (Zumberge et al., 1997). It is capable of PPP processing of the GPS data as well as processing of SAR and DORIS data. In this study seven continuous GPS points of Marmara GPS Network (MAGNET) were used. These points are located at the Marmara region. Figure 2 shows the locations of the seven GPS points used in the processing. Data was collected over a period of three years, between 2002 and 2004.

Figure 2: GPS Points used in this study. (Taken from TUBITAK MAM)

In general, data analysis can be performed in five steps (Sanli, 1999): 1. Preparing (i.e. cleaning and compressing the data for the solution) 2. Forming the design matrix, 3. Applying filtering and least square (LS) estimation, 4. Removing outliers and reprocessing.

TS 3I - Deformation Monitoring – Techniques and Case Studies Asude A. Sørensen and Onur Yilmaz Analysis of Strain Accumulation of the Faulting Zones by the Help of Continuous GPS Stations Integrating Generations FIG Working Week 2008 Stockholm, Sweden 14-19 June 2008

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3.1. Evaluation of Velocity Field The temporary postseismic ground deformation observations after major earthquakes is an important tool to understand the mechanics of the earthquake process. Besides, it is also important to understand the mechanical behavior of the region surround the fault zone. Postseismic deformations can continue from immediately after the earthquake time to several tens of years as a function of time dependent stress relaxation (Ergintav et al., 2002). In this study, the results of GIPSY Software show that at the year of 2002, the velocities of the stations close to the fault are two times as big as the ones which are far away from the fault. At the years 2003 and 2004, it is observed that these velocities are decreasing. Figure 3 shows the displacement vectors which are calculated according to the north (N), east (E), up (U) values by GIPSY. The rate of GPS velocities can be seen in Table1.

Figure 3: Displacements of stations between 2002 and 2004

TS 3I - Deformation Monitoring – Techniques and Case Studies Asude A. Sørensen and Onur Yilmaz Analysis of Strain Accumulation of the Faulting Zones by the Help of Continuous GPS Stations Integrating Generations FIG Working Week 2008 Stockholm, Sweden 14-19 June 2008

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Table 1. Horizontal GPS velocities of the Marmara Region at Eurasian fixed frame

Site

East & North rate East & Longitude (θ) Latitude (θ) (mm/year) (mm/year) ±

ULUT BAD1 BOZT AVCT KART MER1 MADT

29,131 29,118 28,782 28,724 28,333 27,962 27,587

40,098 40,852 40,534 40,989 40,265 40,967 40,611

−23,02 0,29 −15,84 −4,84 −19,69 1,10 −15,86

−2,59 1,20 −2,82 3,11 −2,11 1,87 −3,30

0,30 0,31 0,46 0,41 0,28 0,29 0,37

North 0,28 0,29 0,48 0,42 0,26 0,26 0,38

4. STRAIN As a result of tectonic plates movements, enormous forces are applied to the earth’s crust. These forces change the plate configurations and make some variations in states of stress in the rocks. When stresses are applied to rocks, the rocks may change position and shape. The change in position is known as displacement. Strain is known as the change in shape. In general, strain is used for the determination of deformation. 4.1. Strain Analysis with Geodetic Methods Strain analysis are commonly used for determination of the deformation. The main geodetic methods that are used for strain accumulation are; – adjusted measurements that is; length, angle and azimuth differences, – coordinate differences, – determination of strain parameters in an adjustment model. Table 2: Strain parameters that are defined by repeated geodetic observations

Parameters

Length

Area Deformation (Δ) Shear Strain (γ) Eigenvalues (ε1 , ε2)

+ + +

Rotation Angle (φ) Angular Strain (ω)

+

Azimuth Angle

GPS

+

+

+ + +

+ +

+

+ +

TS 3I - Deformation Monitoring – Techniques and Case Studies Asude A. Sørensen and Onur Yilmaz Analysis of Strain Accumulation of the Faulting Zones by the Help of Continuous GPS Stations Integrating Generations FIG Working Week 2008 Stockholm, Sweden 14-19 June 2008

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4.2. Strain Tensor Strain tensor can be defined by below matrix;

⎡ e xx e xy ⎤ E= ⎢ ⎥ ⎣⎢ e xy e yy ⎦⎥

(1)

The diagonal elements of strain tensor ( e xx e xy ) shows extension through the coordinate axes. The other elements ( e xy ) show the angular deformation that is a result from deformation according to coordinate axes. Strain tensor parameters are determined with least square adjustment method. For strain parameters the below formulas are used: Pure shear: γ 2 = 2e xy

ϕ = 0.5* tan −1 ( γ 2 / γ1 )

(2) (3)

Maximum strain direction:

ϕ = 0.5* tan −1 ( γ 2 / γ1 )

(4)

The infinitesimal strain-displacement relationships can be summarized as, 1 ⎛ ∂ui ∂u j ⎞ + ⎟ 2 ⎝ ∂x j ∂xi ⎟⎠

ε ij = ⎜ ⎜

(5)

Shear strain parameters (γ1, γ2) show the angular deformation (Figure 4.). Pure shear (γ1) is the augmentation of the angle that is between two perpendicular northwest-northeast directions. (γ2) defines decrement between two perpendicular the north and east directions (Feigl, 1990).

TS 3I - Deformation Monitoring – Techniques and Case Studies Asude A. Sørensen and Onur Yilmaz Analysis of Strain Accumulation of the Faulting Zones by the Help of Continuous GPS Stations Integrating Generations FIG Working Week 2008 Stockholm, Sweden 14-19 June 2008

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Figure 4: Shear strain parameters (γ1, γ2).

5. DETERMINATION OF STRAIN FIELD IN THE WESTERN PART OF NORTH ANATOLIAN FAULT ZONE 5.1. Computation of Strain Parameters with Triangulation

For this method the field was separated to six triangles and strain fields of all triangles assumed s homogeneous and the strain parameters of each triangle was calculated (Table 3). Table 3: Strain triangles

Regions

Whole area Triangle 1 Triangle 2 Triangle 3 Triangle 4 Triangle 5 Triangle 6

Point Names

AVCT, BAD1, ULUT, BOZT, KART, MER1, MADT BAD1-BOZT-ULUT BAD1-BOZT-AVCT BOZT-ULUT-KART BOZT-AVCT-KART MER1-AVCT-KART MER1-MADT-KART

Triangles in the network are taken as “unit particle” of finite element method. By combining unit particles continuous parameter in the network (solution area), continuous parameters are found. For each baseline of a triangle, three general equations are created. Thus, are found. exx , exy , e yy Strain parameters which were calculated with this method are independent from datum parameters. Calculated strain parameters for the points of equilibration of triangles show scale differences in the network. It is required to provide the continuity for the network among the parameters which are calculated for points of equilibration. TS 3I - Deformation Monitoring – Techniques and Case Studies Asude A. Sørensen and Onur Yilmaz Analysis of Strain Accumulation of the Faulting Zones by the Help of Continuous GPS Stations Integrating Generations FIG Working Week 2008 Stockholm, Sweden 14-19 June 2008

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Figure 5: Triangles of the field

Calculated strain parameters are shown in Table 4 and extensions or compressions for the whole are is shown in Figure 6. Table 4: Calculated strain parameters

PRINCIPLE STRAIN COMPONENTS

TRIANGLE CORNER POINTS

ε1 (μs)

BAD1-BOZT-ULUT BAD1-BOZT-AVCT BOZT-ULUT-KART BOZT-AVCT-KART MER1-AVCT-KART MER1-MADT-KART

0.5828 0.7782 0,2189 0,4077 0,4102 0,5001

ε2 (μs)

Ψ (deg)

EINTER (μs)

ESHEAR (μs)

-0,0859 -0,3067 -0,5221 -1,395 -0,075 -0,0426

6,9826 30,9725 -89,9999 47,9029 54,0592 40,9145

0,2485 0,2357 -0,1516 -0,4935 0,1676 0,2287

0.3344 0,5425 0,3705 0,9012 0,2425 0,2714

TS 3I - Deformation Monitoring – Techniques and Case Studies Asude A. Sørensen and Onur Yilmaz Analysis of Strain Accumulation of the Faulting Zones by the Help of Continuous GPS Stations Integrating Generations FIG Working Week 2008 Stockholm, Sweden 14-19 June 2008

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Figure 6: Extensions and compressions for whole region accepted as homogeneous

From the figure6, it can be seen that according to the calculations the compression at the BOZT-AVCT-KART triangle is very obvious. 5.2. Computation of Strain Parameters with Infinitesimal Method

This method is based on the measurements coordinate differences between a fixed point and its surroundings. Strain tensor parameters are calculated from adjusted coordinate differences. This method can be applied if the number of connections are more than three. dx and dy coordinate differences or deformation vector for each point can be measured from the general deformation equation.

TS 3I - Deformation Monitoring – Techniques and Case Studies Asude A. Sørensen and Onur Yilmaz Analysis of Strain Accumulation of the Faulting Zones by the Help of Continuous GPS Stations Integrating Generations FIG Working Week 2008 Stockholm, Sweden 14-19 June 2008

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Figure 7: Point P and its surrounding points

dxi = xi e xx + yi e xy + t x

(6)

dyi = xi e yx + yi e yy + t y

(7)

With strain tensor parameters maksimum and minimum strain principle parameters and maksimum strain rotation can be calculated; ε1 =0.5*(e xx +e yy )+ (0.25*(e xx +e yy ) 2 +e xy 2 )

(8) (9)

ε 2 =0.5*(e xx +e yy )- (0.25*(e xx +e yy )2 +e xy 2 )

β=arctan

e xy

(10)

ε1 -e xy

After all calculations were done for all points, the results are shown on table 5. Table 5: Infinitesimal strain parameters

STATION Principle strain parameters ID ε2 (μs) ε1 (μs)

β (deg)

KART MER1 AVCT BOZT ULUT BAD1

263,721 648,121 413,968 603,227 342,705 285,418

-0,16 1,25 5,66 0,22 0,15 0,70

-3,57 -0,48 -0,31 -0,08 0,05 -0,09

TS 3I - Deformation Monitoring – Techniques and Case Studies Asude A. Sørensen and Onur Yilmaz Analysis of Strain Accumulation of the Faulting Zones by the Help of Continuous GPS Stations Integrating Generations FIG Working Week 2008 Stockholm, Sweden 14-19 June 2008

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6. CONCLUSION

In this study, strain accumulation around the fault zones in western Marmara Region is calculated with two techniques as triangulation and infinitesimal strain model. Since the region has very complex tectonic characteristic, the strain parameters are calculated separately. ε1 and ε2 defines the principle strain parameters. Positive values of principle strain parameter refer extension, negative values refer compression. In Figure 6, the arrows show the extension and compression quantities. Additionaly, velocity of the stations at the northern branch of NAF at the Marmara Region have bigger displacements than the stations at the southern branch. REFERENCES

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McClusky, S., Balassanian, S., Barka, A., Demir C., Ergintav, S., Georgiev, I., Gurkan, O., Hamburger, M., Hurst, K., Kahle, K., Kastens, K., Kekelidze, G., King, R., Kotzev, V., Lenk, O., Mahmoud, S., Mishin, M., Nadariya, M., Ouzounis, A., Paradissis, D., Peter, Y., Prilepin, M., Reilinger, R., Sanli, I., Seeger, H., Tealeb, A., Toksöz, M.N., Veis, G., 2000, Global Positioning System constrains on plate kinematics and dynamics in the eastern Mediterranean and Caucasus, Journal of Geophys. Res., Vol. 105, No. B3, pp. 5695–5719. R. Armijo, B. Meyer, S. Navarro, G. King, and A. Barka, 2002, Asymmetric slip partitioning in the Sea of Marmara pull-apart: A clue to propagation processes of the North Anatolian fault?, Terra Nova, 14, pp. 80–86. Reilinger, R. E., Ergintav, S.; Burgmann, R., McClusky, S., Lenk, O., Barka, A., Gürkan, O., Hearn, L., Feigl, K.L., Çakmak, R.; Aktug, B., Özener, H., Toksöz, M. N., 2000, Coseismic and Postseismic Fault Slip for the 17 August 1999, M = 7.5, Izmit, Turkey Earthquake, Science, Vol. 289. Sanli, D.U., (1999), “GPS Straregies for Tide Gauge Monitoring with Assessment of Sea Level Analysis Models”, A Thesis for Degree of Doctor of Philosopy, Department of Geomatics, University of Newcatle upon Tyne. Straub, C., Recent Crustal Deformation and Strain Accumulation in the Marmara Sea Region, N.W. Anatolia, Inferred from GPS Measurements, PhD Thesis, ETH Zürich, Switzerland. 1996 Tüysüz, O., Genç, C., 1999, Geological Factors Controlling the Distribution of Damage During the 17th August and 12th November 1999 Earthquakes, International Conference on The Kocaeli Earthquake, Kocaeli. Üçer, B., Eyidogan, H., Gürbüz, C., Barka, A., Serif, B., 1997, Seismic Investigations of Marmara Region in Active Tectonics of Northwest Anatolia, The Marmara Poly-Project, Hochschulverlag AG an der ETH, Zurich, pp. 55-87. Y.Y. Kagan, Jackson D.D., 1999, Worldwide doublets of large shallow earthquakes, Bull. Seis. Soc. Amer. v.85, no.9. Zumberge, J. F., Heflin, M. B., Jefferson, D. C., Watkins, M. M., and Webb, F. H., 1997. Precise point positioning for the efficient and robust analysis of GPS data from large networks, Journal of Geophysical Research, 102: 5005-5017. CONTACTS:

Asude Arslan Sørensen University of Bergen NORWAY E-mail: [email protected] ; [email protected]

TS 3I - Deformation Monitoring – Techniques and Case Studies Asude A. Sørensen and Onur Yilmaz Analysis of Strain Accumulation of the Faulting Zones by the Help of Continuous GPS Stations Integrating Generations FIG Working Week 2008 Stockholm, Sweden 14-19 June 2008

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